Gary Smith, a plumber who worked for Pimlico Plumbers for five years from 2005, has already won a number of court rulings that determined he could claim “worker” status even though he was described in his contract as “self-employed operative”.

The Court of Appeal said Mr Smith was a worker because he was required to use the firm’s van for assignments and was contractually obliged to do a minimum number of hours a week.

Pimlico Plumbers is appealing against the decision in a hearing likely to last two days, with judgment expected in a few weeks’ time.

Charlie Mullins outside the Supreme Court in London (Victoria Jones/PA)

Mr Mullins said: “Tomorrow we are going to battle in the UK Supreme Court and the outcome of the case will have huge ramifications for a large part of the economy, including the media, the health service, and of course the construction industry of which plumbing and other trades undertaken by Pimlico Plumbers make up a huge part.

“In Isolation the case we are fighting tomorrow started six years ago when a self-employed plumber, Gary Smith, who did work for Pimlico Plumbers decided that despite the fact that he had always known he was a self-employed contractor, decided that he would demand employment rights that only employees, not contractors, are entitled to.

“And now all these years later we find ourselves still fighting to prove that a man who took every advantage of being a self-employed plumber when it suited him is indeed a contractor.”

Mr Mullins said employment law was “unclear”, adding that he hoped to get some clarity from the Supreme Court.

Jason Moyer-Lee, general secretary of the Independent Workers Union of Great Britain, said: “With the rise of the so-called gig economy, the issue of employment status is more important than ever.

“So far, tribunals’ and courts’ interpretations of these issues have done quite well to keep up with the times – that’s why nearly all high-profile gig economy cases have declared the individual to be a worker, and consequently entitled to minimum wage rights and holiday pay.

“I expect the Supreme Court will do the same with the Pimlico Plumbers case.”